The present invention relates to an image forming apparatus such as an electrophotographic copying machine, and more particularly, to an image forming apparatus wherein a toner image obtained through development of a latent image on an image carrier therein is transferred onto a recording sheet through a physical means of a transfer belt device having a rotary transfer belt and then the recording sheet to which the toner image adheres is transported to a fixing means where the toner image is fixed, while the image carrier from which the toner image has been transferred is neutralized electrically by a neutralizer and then is cleaned for the following image formation.
On an image forming apparatus such as an electrophotographic copying machine or the like, the surface of a photoreceptor drum, for example, which is an image carrier is charged evenly by a charging unit and then the surface of the photoreceptor drum is subjected to imagewise exposure for formation of an electrostatic latent image. The latent image is developed by a developing means and visualized to be a toner image. The toner image formed on the circumferential surface of the photoreceptor drum is transferred by a physical means onto a recording sheet which is transported synchronously, and the recording sheet on which the toner image has been transferred is separated from the photoreceptor drum and is transported to a fixing means where the toner image is fixed on the recording sheet. After that, the recording sheet is ejected to the outside of the apparatus.
As a means for transferring the toner image adhering to the circumferential surface of the photoreceptor drum to a recording sheet, a transfer unit that conducts electric discharge on the recording sheet from its back side charges with electricity having polarity opposite to that of toners, the toner image is transferred onto the recording sheet, and the recording sheet is separated from the photoreceptor drum after being neutralized with AC high voltage impressed thereon by a separating unit after the transfer. However, it is difficult to keep for sure the satisfactory conditions of transfer and separation. Especially when the photoreceptor drum has a large diameter, a recording sheet is difficult to be separated and a recording sheet onto which a toner image has been transferred sticks to the photoreceptor drum without being separated, tending to cause sheet jamming, which is a disadvantage. As a technology improving the disadvantage, a transfer belt device is used.
In the transfer belt device, a transfer belt stretched around a plurality of supporting rollers is rotated at the same speed as the photoreceptor drum. The transfer belt device is equipped with a charge-donating means that donates electric charges to a recording sheet so that the recording sheet may be attracted by electrostatic force to the transfer belt, thus the recording sheet is brought into contact with the photoreceptor drum at a transfer section while adhering to the transfer belt, and at the transfer section, high voltage having polarity opposite to that of toners is impressed on the recording sheet under the control of constant current for transferring toner images. Such transfer belt device offers excellent transfer efficiency and excellent separation effect at the transfer section.
For a color image forming apparatus wherein toner images are superimposed on a photoreceptor drum to be transferred onto a recording sheet at once, the transfer belt device mentioned above is used as a device which is especially preferable. In the color image forming apparatus mentioned above, a plurality of developing units are provided around the photoreceptor drum because toner images are superimposed on the photoreceptor, which requires a large drum diameter that needs more positive separation efficiency because sufficient separation efficiency can not be obtained in a conventional electrostatic transfer and separation system. Further, in the system of superimposing toner images, an amount of adhering toners is large and thereby a large amount of electric charges for transfer is required. Therefore, it is necessary to have great capacity for keeping electric charges for transfer. For the two points mentioned above, the transfer belt device is excellent, and therefore, it is used as an especially preferable device.
However, for photoreceptor drum 110 as shown in FIG. 4, corona discharge unit 136 is provided for corona discharge on the back side of transfer belt 131 at nip portion 135 to which the transfer belt 131 stretched around supporting rollers 132 and 134 touches, then, toner images formed on the photoreceptor drum 110 are transferred onto recording sheet sticking to and transported by the transfer belt 131, and after the recording sheet from which images have been transferred is separated from the photoreceptor drum 110, electric charges from corona discharge unit 136 are transferred onto the photoreceptor drum 110 from which the recording sheet has been separated to cause the photoreceptor drum to be in a charged state. When the photoreceptor drum 110 is superordinate for charging, it is not possible to remove sufficiently the toners remaining on and sticking to the photoreceptor drum 110 after transfer even when cleaning means 116 is used for removing them, which is a problem. Therefore, neutralizer 115 is provided to impress AC bias voltage for neutralizing.
An effect of the neutralizer 115 provided can be confirmed in cleaning. According to the study of the inventors of the invention, however, the photoreceptor drum subjected to AC neutralizing by means of simple AC bias voltage and still holding remaining toners thereon is not necessarily cleaned fully by the following cleaning means. Namely, there is observed a phenomenon that a part of remaining toners removed once from the surface of the photoreceptor drum by a cleaning blade in cleaning is scraped up to the tip of the cleaning blade and sticks to the surface of the photoreceptor drum again after being redeveloped. This phenomenon is remarkably observed on a color image forming apparatus of a multi-color type wherein toner images each having a different color are superimposed and formed on the surface of the photoreceptor drum and then are transferred onto a recording sheet. The reason for the phenomenon that the remaining toners cause redeveloping is considered as follows: with regard to electric charges given to the photoreceptor and toners by the corona discharge unit 136, those given to the remaining toners which are in the state of adhering are not neutralized fully by AC neutralization although those given to the photoreceptor whose back side is grounded are neutralized to be zero in voltage by AC neutralization. A phenomenon of redeveloping is observed remarkably at a point of time of pressure-contact and releasing of the cleaning blade.
In a color image forming apparatus having the multi-color function, it is not possible to keep causing the cleaning blade of cleaning means 116 to be in pressure-contact when forming a color image because toner images each having a different color are superimposed and formed on the circumferential surface of photoreceptor drum 110. Namely, after transferring the precedent color images and before forming the following color images, the cleaning blade is brought into pressure-contact for cleaning the circumferential surface of the photoreceptor drum 110, and then the pressure-contact of the cleaning blade is released for preparation for the following color image forming.
Toners used for redeveloping during the aforementioned releasing of the cleaning blade stay again within a region related to image forming, resulting in an adverse effect on color images.
In the case of a mono-color image forming apparatus (not illustrated), on the contrary, it is possible to keep causing the cleaning blade to be in pressure-contact with an entire region related to image forming because it is not necessary to superimpose toner images each having a different color on the circumferential surface of the photoreceptor drum. Therefore, it does not happen that redeveloping caused by pressure-contact and releasing of the cleaning blade affects adversely on images. Further, an amount of residual toners is less on the mono-color image forming apparatus than on the color image forming apparatus of a multi-color type. Therefore, redeveloping does not affect adversely on images in the case of the mono-color image forming apparatus.
On an image forming apparatus of a multi-color type, therefore, neutralizing voltage for neutralizer 115 is made to be those wherein D.C. voltage that is the same in polarity as transfer voltage and is appropriate in voltage is superposed on A.C. bias voltage, so that there may be no difference in voltage between photoreceptor drum 110 and residual toner, for excellent cleaning.
Even in the case of a color image forming apparatus having the multi-color function wherein a plurality of developing units are provided around a photoreceptor drum and toner images each having a different color are superimposed and formed on the surface of the photoreceptor drum after repetition of the process of charging, imagewise exposure and developing and then are transferred onto a recording sheet, it is not necessary to redevelop on a multi-color basis when images on a document are characters. In addition to a multi-color mode, therefore, there is provided a mono-color mode wherein only one of a plurality of developing units each containing different color developer is operated and a process including charging, imagewise exposure, developing and transfer that immediately follows the developing is taken. In the case of a multi-color mode, it is not possible to form a multi-color image whose length exceeds one circumference of a circle of the photoreceptor drum but is possible, in the case of a mono-color mode, to record an image whose size exceeds one circumference of a circle of the photoreceptor drum as shown in FIGS. 2 (a) and 2 (b). In FIG. 2 (a), the numeral 10 represents a circumferential surface of the photoreceptor drum, and dotted line portions D (A)-D (C) represent regions where images were formed on the circumferential surface of the photoreceptor drum, while ranges D (B)-D (C) represent a portion where images were formed twice. FIG. 2 (b) shows a recording sheet on which P (A) corresponds to D (A) in image forming and P (C) corresponds to D (C).
In this case, when corona discharge impressed by a neutralizer is conducted under the same condition as in a multi-mode in recording an image whose size exceeds one circumference of a circle of the photoreceptor drum, ranges D (B)-D (C) where image forming was conducted twice for one image sometimes show thereon a photographic fog.
The reason for the foregoing is as follows. Since neutralizing voltage in the multi-color mode is one wherein D.C. voltage that is the same in polarity as transfer voltage and is appropriate in voltage is superposed on A.C. bias voltage, the D.C. voltage that is opposite in polarity to charging voltage affects adversely on charging characteristics. Ranges where image forming is conducted twice on the circumferential surface of the photoreceptor drum for one image, in particular, show remarkable adverse effect on charging characteristics caused by bias voltage of a neutralizer. Therefore, occurrence of photographic fog is observed on the region where image forming is conducted twice for one image.
An object of the invention is to provide an image forming apparatus capable of forming an excellent image under a mono-color mode as well as a multi-color mode.
Further object of the invention is to provide an image forming apparatus capable of forming an excellent image independently of the size of an image to be recorded.